Modern process plants, designed for flexible production and to maximize recovery of energy and material, are becoming more complex. Advanced control can improve product yield; reduce energy consumption; increase capacity; improve product quality and consistency; reduce product giveaway; increase responsiveness; improve process safety and reduce environmental emissions.
By implementing advanced control substantial reductions in operating costs can be obtained. These benefits are clearly enormous and are achieved by reducing process variability, hence allowing plants to be operated to their designed capacity. Process units are tightly coupled and the failure of one unit can seriously degrade overall productivity. This situation presents significant control problems. However, it is generally acknowledged that there is currently not one technique that will solve all the control problems that can manifest in modern plants. Indeed, different plants have different requirements.
Programmable logic controllers, commonly referred to as PLCs and also known as industrial machine controllers offer one methodology for process control. Programmable logic controllers have been in use for decades and have proven reliability in harsh environments and are designed to handle many inputs and outputs which has made them the foundation of many factory automated systems.
PLCs are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the system in accordance with a stored control program. In programmable controllers the control program is stored in a random access memory and includes sets of instructions which are executed in rapid sequence to examine the condition of selected input devices on the controlled system and instructions which energize or de-energize selected output devices on the controlled system contingent upon the status of one or more of the examined input devices. The response of the controlled system to changing conditions is directly related to the time it takes to scan the entire control program, and because the control program is executed at a fixed rate, the time necessary to cycle through it is a constraint on the complexity of the system which can be controlled by the programmable controller.
PLCs are typically well adapted to turning outputs on or off based on the state of inputs. They are also well adapted to bringing together and concentrating a lot of data and status that is uploaded into a computer in a compact form and PLCs are generally more rugged than computers.
Conversely, PLCs are not the best at handling large amounts of data, complex data, or advanced math functions. In addition, when purchasing a PLC the objective is to purchase the input/output module that works with the PLC and that matches the actual signal/device being connected to the PLC. A PLC input/output module can also be purchased with or without a CPU and the input/output module can be utilized. PLC input/output modules are typically well designed and can be connected to a computer controller with a communications link between the computer and the PLC input/output module. This obviously requires the purchase of additional hardware which increases acquisition and implementation costs and eventually maintenance costs.
Another option available to those seeking to control industrial processes is the purchase of an industrial computer or what is commonly referred to as a Soft PLC. Industrial computers are; however, often two to three times the cost of office grade computers. In some cases, such as limited space, a user does not have an option and must use an industrial computer. The biggest drawback to industrial computers is that since they are replaced as often as office computers (typically every three years) costs are double or triple. To overcome this cost increase it is possible to use an industrial monitor, mouse, keyboard, office computer and relocate the computer to a less hazardous environment. It is possible, with additional cost, to use monitors and keyboard extenders, Ethernet, and other tricks to extend the distance between the computer and industrial environment. This obviously can create difficulties if having the controls in proximity to the process is beneficial for purposes of process troubleshooting. Regardless of whether an office or industrial computer is used it is still essential to protect the computer with surge suppression, noise filtering, vibration isolation, uninterruptible power supply, cooling etc., all of which adds additional cost and maintenance concerns.
A PLC operates with embedded firmware while a Soft PLC utilizes a commercial operating system. Soft PLCs are considered by many to be inherently unstable because they typically utilize third party software that has not been tested as part of a control system where as programmable logic controllers and distributed control systems have typically undergone extensive testing by vendors or third party consultants to validate their capabilities.
Another option for process control is a distributed control system (DCS) which utilize a mainframe to control multiple processes. A distributed control system (DCS) is part of a manufacturing system. It is generally digital, and normally consists of field instruments, connected via wiring or busses to multiplexers/demultiplexers and analog to digital and finally the human machine interface or control consoles. Distributed control systems (DCS) are used in industrial and civil engineering applications to monitor and control distributed equipment with remote human intervention. DCS is a very broad umbrella that describes solutions across a large variety of industries, including: electrical power distribution grids and generation plants, environmental control systems, traffic signals, water management systems, and refining and chemical plants. The broad architecture of a DCS solution involves either a direct connection to physical equipment such as switches, pumps and valves or connection via a secondary system such as a supervisory control and data acquisition system (“SCADA”). A DCS solution does not require operator intervention for its normal operation, but with the line between SCADA and DCS merging systems claiming to offer DCS may actually permit operator interaction via a SCADA system. The drawback to the DCS is that it is typically very expensive to install and to maintain.
It is clear from industry trends that industrial process control is a vital issue and one whose importance will only increase. Competition from not only within the U.S., but globally, is forcing manufacturers to reduce their production costs through process improvement including greater precision, faster production, less scrap and rework and reduced manual involvement in the process itself.
Presently, none of the technologies identified above provides the optimal combination of low acquisition, installation and maintenance costs and yet provides the robust capabilities of a microcontroller controlled process that is optimally positioned in close proximity to the process itself and that can readily be reprogrammed to accommodate process variations. Therefore new systems and methods are needed.